I'm schemin' and dreamin'...my next speaker project...I have to ask:
Will placing the M's in an mtm on different planes improve dispersion (energy distribution) and be a valid addition to the project? What are other reasons to consider this?

I reference speakers such as Wilson's Maxx or Alexandria and JMLab's Utopia series

JMLab marketing hype says something about putting the drivers the same distance from the listener. I take it that means some sort of phase alignment without the diffraction effects of a recessed tweeter got lost in translation to marketing speak. It seemed to work in my micro utopia inspired case - no phase alignment was required in the active XO.

Another consideration is that you get a quicker rolloff on the system axis with the mids off axis. this may allow more cconvenient integration.

I've also noticed that some drivers Focal's 6W4311 for example, show less of a breakup peak off axis. This could eliminate teh need for a notch filter.

I am not sure what the speakers you reference look like, but the agreement I have seen for having speakers on the same baffle but different planes is that the point in the speaker the generates the sound are all on the same plane. A tweeter is swallower then a big woofer, so if the sound generation point of both speakers are on the same plane, then the signal is generated from the same plane and are "in-time" with each other.